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Nature Materials (2019)

The ground exciton state of formamidinium lead bromide perovskite nanocrystals is a singlet dark state

Perovskite quantum dots (QDs) are promising nanostructures for optoelectronics, especially for the application to light emitting devices (LED). CsPbX3 inorganic QDs or FAPbX3 hybrid QDs (X=I, Br, Cl and FA= formamidinium) have exhibited an exceptional brillancy and high quantum yields. Two CNRS teams (LP2N-Bordeaux et FOTON-Rennes) work together on this subject since 2017. They have previously studied and the exciton bright triplet fine structure in CsPbBr3 single QDs in terms of structural distortions, predicting as well a singlet dark state located below the bright triplet state (Fu et al, Nanoletters 2017, DOI : 10.1021/acs.nanolett.7b00064). Several other groups using time resolved spectrocopy have attributed the brillancy of perovskite QDs to a slow relaxation from the bright triplet to the dark singlet, related according to the joint LP2N/FOTON team in the case of FAPbI3 QDs (Fu et al, Nature communications 2018, DOI : 10.1038/s41467-018-05876-0) to a second order mechanism assisted by two optical phonons. The publication in the journal Nature of a paper in 2018 led to a controversy. Based essentially on a theoretical model, the brillancy of CsPbX3 QDs was attributed to a pure Rashba effect supposedly leading to a clear shift of the bright triplet below the dark singlet. This unprecedented situation extended more generally to all the perovskites as claimed by the same authors, shall put forward them as a very special class among all the other semiconductors with many theoretical and practical implications.

Within this context, the joint LP2N/FOTON team associated to ETH Zurich and EMPA in Switzerland has directly evidenced for the first time the singlet dark excitonic state in perovskite QDs in a paper published by the prestidigious review Nature Materials. The spectroscopy of FAPbBr3 single QDs was performed at low temperature (5K) under magnetic field, to reveal unambigously the presence of a dark singlet state several meV below the dark triplet. This difference is consistent with a theoretical model attributing the splitting essentially to the long-range part of the exchange interaction in a weak quantum confinement regime. This new result is therefore strongly questionning the general scenario purely based on Rashba effect, being moreover fully compatible with previous interpretations. The slow relaxation from the bright triplet to the dark singlet is again related to a second order mechanism assisted by two optical phonons. This article has been further highlighted also in Nature Materials by a special ‘News&views’ (DOI : s41563-019-0376-6).

Jacky EVEN
+33 2 23 23 82 95

LP2N (Bordeaux, France), EMPA (Dübendorf, Suisse), ETH Zurich (Zurich, Suisse)

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